Mixer tracking capacitor



Nov. 22, 1949 L. J. LADER 2,488,545

MIXER TRACKING CAPACITOR Filed Feb. 1, 1946 LLI 3 11:13:. 3 E c BCAPACITANCE INVENTOR.

LEON J4 LADER ATTORNEY Patented Nov. 22, 1949 UNITED STATES PATENTOFFICE (G ant d, unde t act f ar h 3. 883, as amended April 30, 1928;370 G. 757

4 Claims.

This nvention relat s to r ab e condensers and; more particularly to anovel condenser tuning assembly for use ultra-.high-frequency apparatus-At frequencies in the ultra-high-frequency regions of the radiospectrum, transmission lines of the two-wire or coaxial type andresonant cavities are commonly used as resonant circuits. Whentransmission lines and associated thermionic tubes are used atultra-high-frequencies, the inter-electrode impedances of the tubes havea marked effect on the results obtainable. The natural capacity betweenthe various electrodes, as well as electrode leads, offer large reactiveeffects at ultra-high-frequencies. These effects render very difficultthe proper matching of tuned circuits as, for example, those associatedwith the heterodyne oscillator and mixer stages of a receiver.

In an ultra-high-frequency superheterodyne receiver it is necessary forproper operation that the heterodyne oscillator frequency track themixer frequency with a constant frequency difference equal to theintermediate frequency of the receiver. The resonant circuits of bothoscillator and mixer consist of similar coaxial line arrangements, theoscillator havin in addition a thermionic tube electrically connected toone end of the oscillator coaxial line resonant circuit, The reactiveeffects of the inter-electrode capacitances of the oscillator thermionictube change the electrical characteristics of the oscillator line sothat at various frequencies the two lines are no longer electricallysimilar and are separated by a frequency other than the intermediatefrequency of the receiver. In accordance with one embodiment of theinvention the coaxial transmission line mixer is caused to trackproperly and simultaneously the frequency changes of the heterodynethermionic tube oscillator, a common control device being used.

Another object of the invention is to provide a novel condenser assemblyfor use as a tuning or compensating condenser with coaxial transmis-.sion lines.

The nature of the present invention together with other objects andfeatures thereof will appear more fully in the following description ofa specific embodiment, reference being made to the appended drawing, inwhich:

Fig. 1 illustrates in perspective the condenser assembly of theinvention;

Fig. 2 shows in perspective an ultra-high-frequency mixer and oscillatoremploying the new QQIIdenser assembly;

Fig. 3 is a curve showing the variation of fre-. quency with capacitychange of the subject condenser.

Referring now to Fig. 1 in more detail, the condenser assembly thereshown consists of a disk or flat circular plate I having a sector ofincreased radius l4 and mounted on a shaft 2, the disk being therotatable or rotor part of the condenser. The non-conducting shaft 2 isso mounted that the rotor plate will easily turn when the shaft isturned by manual or mechanical means. A special-shaped block 3 of brassor other conducting material, having a curved face on the side nearestthe rotor and mounted in the same plane as the rotor forms thestationary grounded plate or stator of the condenser. The position ofthe stator block 3 may be changed or varied in the plane of the rotorand in a direction tangent to the curved face at the ends of the blockso that the spacing between the rotor and stator plates of the condensermay be varied. At some position, arbitrarily designated as position B,the curve on the condenser face of the stator block is that of a ninetydegree are of a circle with its center at the center of the shaft 2. Thearrow in Fig. 1 designates the direction of motion, the stator beingshown in position B.

The stator block may be held in position by any of several means,probably the simplest method being shown in the drawings. Two or morescrews 4 pass through a slot in the mounting or base plate 5 of thestator and turn into tapped holes in the stator. By loosening thescrews, the stator can be slid back and forth to any desired positionand then looked in place by tightening the screws.

The tuning position of the rotor is with the sector of increased radiusIt turned toward the stator block, and extends through thatapproximately ninety degrees of arc on the curved face of the statorblock. When the stator block is fastened in position B, or that positionin which the centers of curvature of both the face of the stator blockand the rotor plate are coincident, there will be no change incapacitance between the rotor and stator block as the rotor is turnedthrough its normal ninety degree tuning range. When connected inparallel with another variable condenser as is the usual procedure, thenet total capacitance during tuning will be that of the usual condenserplus a constant fixed amount depending on the structure of the subjectcondenser. It is known in the art that the capacitance between twoplates is directly proportional to the common area presented by the twoplates and inversely proportional to the distance between them.Therefore, the larger the tooth or sector of increased radius Id of therotor, the greater is the maximum capacity of the condenser. Fig. 3shows the effect of tuning in curve B when the usual condenser is onehaving linear frequency-capacitance characteristics.

When the stator block is moved closer to the rotor, the tooth or sectorof increased radius 14 of the rotor is no longer equidistant from theface of the stator block throughout the ninety degree turn. When thetooth [4 lies parallel to the direction of movement of the statorblock, 1. e., pointing toward the top of the stator as shown in Fig. 1,the spacing between stator and rotor is a minimum, hence the capacity ismaximum. As the rotor is turned through its ninety degree range, thespacing increases, thus decreasing the capacitive effect of thecondenser. This effect is shown in curve A of Fig. 3. Similar ly, if thestator block is moved away from the rotor, the maximum spacing, henceminimum capacity, is found when the rotor tooth is parallel to thedirection of movement of the stator block. Therefore, as the rotor isturned, the spacing will decrease and the capacity will increase. Thischaracteristic is shown in curve C of Fig. 3.

Referring to the curves of Fig. 3, all are seen to intersect at a point0. This is the point where the capacitance of all curves issubstantially equal, occurring when the rotor tooth is at right anglesto the direction of movement of the stator block. The curves in Fig. 3are shown, for explanatory purposes only, to intersect at point 0. It isto be understood that the points of intersection of the curves may varyslightly from a condition of exact coincidence. If, however, the lowerend of the stator block extended tangentially to the curvature thereof,and the movement of the stator block, with respect to the rotor toothwhen at right angles to the direction of movement, were to be limited tothe extended portion, all curves would theoretically intersect at point0. For a practical working capacitor constructed in accordance with thepresent invention, the deviations of the curves from point is so slightas to be negligible, therefore eliminating the need for the extendedportion of the lower end of the stator block. Hence, it can be seen thatthe subject condenser acts to change the slope of thefrequency-capacitance curve when used in parallel with anothercondenser. When the stator block is in position B, the characteristic ofthe curve is that of the standard paralleling condenser. In position Cthe capacitance increases less rapidly than in position B as the rotoris turned from the point 0, and in position A the capacitance increasesmore rapidly.

A practical application of this invention is shown in Fig. 2, describinga mixer-oscillator assembly. The problem involving the difficulties ofproper tracking between mixer and oscillator stages in anultra-high-frequency receiver has been stated earlier. To overcome thesedifiiculties the subject condenser is used. The assembly consists of tworesonant line sections, a coaxial mixer chamber having a hollowrectangularshaped outer conductor 6 and a solid rectangular innerconductor 8, and a coaxial oscillator chamber having a hollowrectangular-shaped outer conductor 9 and a solid rectangular innerconductor Ill. One end of the oscillator chamber is open, the other endbeing connected to an electronic tube which is not shown in the drawingsince it does not form a part of the invention. One end of the mixerinner conductor is electrically connected to the outer conductor at [5through a short-circuiting plate, and the other end is open. The tworectangular chambers are placed side by side with the open ends togetherand a single insulating shaft 2 passes through the two outer conductors,but off the ends of the inner conductors. One end of the shaftterminates in a bearing of suitable type; the other end is connected toa knob H or other turning means. Mounted on the shaft in the oscillatorchamber are two standard condenser rotor plates I2 and 13. These platesare disk-shaped with a rectangular boss or raised area It on the face ofthe disk extending from the edge of the plate to beyond the center ofthe disk on the opposite side of the same face. Midway between thecenter and the edge the disk is cut off, the cut being square with theboss. The plates are positioned with the bosses facing the innerconductor, one on either side of the inner conductor. so that acapacitive effect is obtained between the rotor plates and the innerconductor and the rotor plates and the outer walls of the chamber.Fastened on the shaft in the mixer chamber is one standard rotor plate 1and the special rotor plate I of this invention, one on either side ofthe inner conductor 8. Facing the special rotor l in its proper plane isthe movable stator block 3. The two rotors in the mixer chamber alsohave a capacitive effect to the inner conductor and to the walls of thechamber. Energy is coupled by any suitable known method from theoscillator cavity to the mixer, from the mixer to the first intermediatefrequency stage, and from the incoming signal source to the mixer.

The oscillator is tuned through its normal oscillating range by varyingthe capacitive coupling between the inner conductor plate line and theouter conductor. The frequency capacitance characteristic curve of theoscillator is essentially a straight line in its usable portion. Hence,in order to obtain the desired intermediate frequency, the mixer signalmust be displaced by a constant frequency from the oscillator frequency, or the mixer frequency must lie parallel to the oscillatorfrequency throughout the tuning range. As stated before, it is verydifiicult to construct an assembly using conventional equipment whichwill accomplish this aralleling of frequencies, but by using the subjectcondenser, proper tracking may be obtained.

Referring again to Fig. 3, the common frequency point 0 is adjusted todesired value by changing the relative phase of the rotors I, I2 and 13of the mixer and oscillator condensers. The other end of the curves,hence the slope of the curves, is determined by moving the groundedstator block 3 through its various and infinite positions. When theproper position of the stator block is obtained, or that one which willgive parallel tracking over the oscillator tuning range, the holdingscrews 4 which are hidden in Fig. 2, are tightened, fastening the statorblock in place.

Thus an ultra-high-frequency device has been described which forms acondenser having novel operating characteristics and which may be usedto improve the performance of much ultra-highfrequency equipment. It hasbeen shown, for example, that when used in conjunction with a receivermixer circuit, the mixer chamber will resonate at the frequency of theincoming signal, and the resonant frequency of the mixer will follow theoscillator frequency at a fixed difference, producing a constantintermediate frequency output. It is obvious that this invention may beapplied equally well to other ultra-highfrequency transmitting andreceiving circuits.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed is:

1. A variable condenser comprising a diskshaped part rotatable about itsaxis of construction with a sector of extended radius, a grounded statorblock placed in the same plane as and opposing the rotor, said statorbeing a block of conducting material having a curved face on the sidenearest the rotor, the curve being a ninety degree are of a circle withits center coincident with that of the rotor when the stator is placedin one arbitrary position, said stator having a means for adjustablyfixing its location with respect to the rotor, the movement being in thesame plane as the rotor but in a direction tangent only to the lower endof the curve of the stator block.

2. A variable condenser, comprising a diskshaped part rotatable aboutits axis of construction with a sector of extended radius, a statorblock including an integrally formed 90 concaved surface cut therein,said block placed in same plane as and opposing the rotor with itsconcaved surface in capacitative relation to the periphery of the rotor,and means for fixing the stator in relation to the rotor the movementbeing in the same plane as the rotor but in a direction tangent only tothe lower end of the curve of the stator block.

3. A variable condenser comprising a rotatable part of a disk-shapedconstruction with a sector of extended radius, said rotor being mountedon an insulating shaft, the shaft passing through the center of therotor and being rotatable during tuning, a grounded stator blockincluding an integrally formed 90 concaved surface cut therein, saidblock placed in the same plane as and opposing the rotor with itsconcaved surface in capacitative relation to the periphery of the rotor,said stator block having means for adjustably fixing its location in theplane of the rotor but in a direction tangent only to the lower end ofthe curve of the stator block to produce a differential capacitativerelationship between the rotor and the extremities of the concavedsurface of the stator.

4. In combination a resonant line oscillator section comprising a flatsolid rectangular inner conductor centrally positioned within a hollowrectangular outer conductor and a resonant line mixer section comprisinga flat solid rectangular inner conductor centrally positioned within ahollow rectangular outer conductor, the sections being so disposed inrelation to each other to be individually but simultaneously tuned bycondenser plates rotatably mounted on a common shaft, said plates insaid oscillator section being mounted one on either side of said fiatsolid rectangular inner conductor within said oscillator section, saidplates in said mixer section also being mounted one on either side ofsaid flat rectangular inner conductor within said mixer section; eitherside of said fiat solid inner rectangular conductors and the inner wallsof said hollow rectangular conductors forming the other plates of theindividual capacitor elements, one of said plates in said mixer sectioncomprising a disk-shaped part rotatable about its axis of constructionwith a sector of extended radius, a grounded stator block placed in thesame plane as and opposing the rotor, said stator being a block ofconducting material having a curved face on the side nearest the rotor,the curve being a ninety degree are of a circle with its centercoincident with that of the rotor when the stator is placed in onearbitrary position, said stator block being movably positioned along thebottom of the mixer hollow rectangular outer conductor and having ameans for adjusting and fixing said stator in any desired position.

LEON J. LADER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,309,455 Herzog J an. 26, 19432,435,140 Koch Jan. 27, 1948 FOREIGN PATENTS Number Country Date 581,666France Oct. 1, 1924

